/*

 * Licensed to the Apache Software Foundation (ASF) under one

 * or more contributor license agreements.  See the NOTICE file

 * distributed with this work for additional information

 * regarding copyright ownership.  The ASF licenses this file

 * to you under the Apache License, Version 2.0 (the

 * "License"); you may not use this file except in compliance

 * with the License.  You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

package com.bff.gaia.unified.sdk.transforms;



import com.bff.gaia.unified.sdk.coders.CannotProvideCoderException;

import com.bff.gaia.unified.sdk.coders.Coder;

import com.bff.gaia.unified.sdk.coders.CoderRegistry;

import com.bff.gaia.unified.sdk.coders.KvCoder;

import com.bff.gaia.unified.sdk.values.KV;

import com.bff.gaia.unified.sdk.values.PCollection;

import com.bff.gaia.unified.sdk.values.TypeDescriptor;

import com.bff.gaia.unified.sdk.transforms.windowing.WindowFn;



import javax.annotation.CheckForNull;

import javax.annotation.Nullable;



import static com.bff.gaia.unified.vendor.guava.com.google.common.base.Preconditions.checkNotNull;



/**

 * {@code WithKeys<K, V>} takes a {@code PCollection<V>}, and either a constant key of type {@code

 * K} or a function from {@code V} to {@code K}, and returns a {@code PCollection<KV<K, V>>}, where

 * each of the values in the input {@code PCollection} has been paired with either the constant key

 * or a key computed from the value.

 *

 * <p>Example of use:

 *

 * <pre>{@code

 * PCollection<String> words = ...;

 * PCollection<KV<Integer, String>> lengthsToWords =

 *     words.apply(WithKeys.of(new SerializableFunction<String, Integer>() {

 *         public Integer apply(String s) { return s.length(); } }));

 * }</pre>

 *

 * <p>Each output element has the same timestamp and is in the same windows as its corresponding

 * input element, and the output {@code PCollection} has the same {@link

 * WindowFn} associated with it as the input.

 *

 * @param <K> the type of the keys in the output {@code PCollection}

 * @param <V> the type of the elements in the input {@code PCollection} and the values in the output

 *     {@code PCollection}

 */

public class WithKeys<K, V> extends PTransform<PCollection<V>, PCollection<KV<K, V>>> {

  /**

   * Returns a {@code PTransform} that takes a {@code PCollection<V>} and returns a {@code

   * PCollection<KV<K, V>>}, where each of the values in the input {@code PCollection} has been

   * paired with a key computed from the value by invoking the given {@code SerializableFunction}.

   *

   * <p>If using a lambda in Java 8, {@link #withKeyType(TypeDescriptor)} must be called on the

   * result {@link PTransform}.

   */

  public static <K, V> WithKeys<K, V> of(SerializableFunction<V, K> fn) {

    checkNotNull(

        fn, "WithKeys constructed with null function. Did you mean WithKeys.of((Void) null)?");

    return new WithKeys<>(fn, null);

  }



  /**

   * Returns a {@code PTransform} that takes a {@code PCollection<V>} and returns a {@code

   * PCollection<KV<K, V>>}, where each of the values in the input {@code PCollection} has been

   * paired with the given key.

   */

  @SuppressWarnings("unchecked")

  public static <K, V> WithKeys<K, V> of(@Nullable final K key) {

    return new WithKeys<>(value -> key, (Class<K>) (key == null ? Void.class : key.getClass()));

  }



  /////////////////////////////////////////////////////////////////////////////



  private SerializableFunction<V, K> fn;

  @CheckForNull

  private transient Class<K> keyClass;



  private WithKeys(SerializableFunction<V, K> fn, Class<K> keyClass) {

    this.fn = fn;

    this.keyClass = keyClass;

  }



  /**

   * Return a {@link WithKeys} that is like this one with the specified key type descriptor.

   *

   * <p>For use with lambdas in Java 8, either this method must be called with an appropriate type

   * descriptor or {@link PCollection#setCoder(Coder)} must be called on the output {@link

   * PCollection}.

   */

  public WithKeys<K, V> withKeyType(TypeDescriptor<K> keyType) {

    // Safe cast

    @SuppressWarnings("unchecked")

    Class<K> rawType = (Class<K>) keyType.getRawType();

    return new WithKeys<>(fn, rawType);

  }



  @Override

  public PCollection<KV<K, V>> expand(PCollection<V> in) {

    PCollection<KV<K, V>> result =

        in.apply(

            "AddKeys",

            MapElements.via(

                new SimpleFunction<V, KV<K, V>>() {

                  @Override

                  public KV<K, V> apply(V element) {

                    return KV.of(fn.apply(element), element);

                  }

                }));



    try {

      Coder<K> keyCoder;

      CoderRegistry coderRegistry = in.getPipeline().getCoderRegistry();

      if (keyClass == null) {

        keyCoder = coderRegistry.getOutputCoder(fn, in.getCoder());

      } else {

        keyCoder = coderRegistry.getCoder(TypeDescriptor.of(keyClass));

      }

      // TODO: Remove when we can set the coder inference context.

      result.setCoder(KvCoder.of(keyCoder, in.getCoder()));

    } catch (CannotProvideCoderException exc) {

      // let lazy coder inference have a try

    }



    return result;

  }

}